fix: copy object missing prefix on metadata delete

When using the REPLACE directive, we were incorrectly removing the
old metadata on the object due to missing the metadata prefix on
the key.  Fix this to remove the correct metadata before setting
new metadata.

Fixes #787

backend/posix/posix.go

@@ -2038,8 +2038,9 @@ func (p *Posix) CopyObject(ctx context.Context, input *s3.CopyObjectInput) (*s3.
 			return &s3.CopyObjectOutput{}, s3err.GetAPIError(s3err.ErrInvalidCopyDest)
 		}
 
-		for key := range mdmap {
-			err := p.meta.DeleteAttribute(dstBucket, dstObject, key)
+		for k := range mdmap {
+			err := p.meta.DeleteAttribute(dstBucket, dstObject,
+				fmt.Sprintf("%v.%v", metaHdr, k))
 			if err != nil && !errors.Is(err, meta.ErrNoSuchKey) {
 				return nil, fmt.Errorf("delete user metadata: %w", err)
 			}